The present invention relates to filter media and more particularly to a unique and novel arrangement for further improving the efficiency of particulate removal and extending the life of the filter resulting in improved filter media performance.
Generally, filter design has balanced a trade off between high filter efficiency and extended life of the filter. Extended life is typically defined according to a limiting pressure drop across the filter or across a thickness in the filter media. The filter will typically be considered to have reached its dust holding capacity when the pressure gradient across the filter has reached this limit.
Efficiency is the propensity of the filter media to capture particulates of any size of particulate matter. Therefore, the more efficient a filter media is at removing particulates from a gas flow stream the more rapidly the filter media will load and approach the holding capacity of the filter. Hence, improving life of filter and efficiency in a filtration media without substantially increasing the pressure drop across the filtration media remains a challenge in improving filtration performance. The term performance as used herein incorporates extended life, efficiency, and pressure drop across the filter wherein performance increases with an increase in efficiency and life of filter.
Varying density depth media arrangements have been developed in an attempt to increase filter media performance. Some such arrangements are described, for example, in U.S. Pat. Nos. 4,082,476; 5,238,474; and 5,364,456. In general, a depth media arrangement can be designed to provide “loading” of particulate materials more uniformly throughout its volume or depth. However, in general, many of these prior designs fail to provide uniform loading and a desired efficiency.
It is also known that filtration efficiency of fibrous filter media can be improved by a significant amount when fibers in a fibrous media are electrically charged, making the fibrous media an “electret”. In 1980, Kubik et al. (U.S. Pat. No. 4,215,682) describes a method for introducing an electric charge into meltblown microfibers during fiber formation.
It has been recognized that there is a critical need in the fluid filtration art to provide filtration media with extended life and with finer particle filtration capabilities without a substantial increase in pressure drop across the filtration media.